In recent years, a silicon photonics technology has been developed. In this silicon photonics technology, an optical signal transmission line made of silicon as a material is formed on a semiconductor substrate, and by integrating various optical devices and electronic devices formed by the optical signal transmission line, a semiconductor device serving as an optical communication module is realized. Among the semiconductor devices of this type, there is an optical signal transmission line provided with an optical waveguide composed of a semiconductor layer formed on a substrate with an insulating layer interposed therebetween and an insulating film formed on the insulating layer so as cover the optical waveguide. In this case, the optical waveguide functions as a core layer, and the insulating layer and the insulating film function as cladding layers.
Japanese Patent Application Laid-Open Publication No. 2012-27198 (Patent Document 1) has disclosed a technique in which in an optical semiconductor device, a semiconductor layer made of an intrinsic semiconductor that is formed on a substrate, and an optical waveguide corresponding to a part of the semiconductor layer are provided.